He was like that before going to Kinderheim from what I recall, so I believe it was Urasawa's symbolism of nature VS nurture, with Johan representing the former. Makes sense if you attribute Nina to the latter. People can seem to naturally have such "evil" tendencies mixed with charisma if taking a look at a few serial killers with psychopatnic or sociopathic tendencies, with a select few even displaying such prior to any kind of trauma. Johan was likely inspired by a few of these.
Explaining how he could practically get out of 99% of situations though...yeah, that was pretty bullshit. Still more believable than Friend or Kenji's Over 9000 level of charisma in 20th CB though.